Importing PNG series with Import Image as Plane is blurred and has artefacts
Nice default settings are to make sure you have Z Transparency as shown below in the Alpha Mode group.. also tick Shadeless as this makes them insensitive to light or shadow and will make them not appear blurry when rendered. You can enable this before or after importing the image(s).



The images come in clean with the alpha setup nicely. As you can see, they can also be layered etc, they might look a bit aliased in the 3d viewport but when rendered show very nicely.

I think the artefacts you might have seen is because they are intersecting with each other, try spacing them apart a bit on the Z axis etc..

These are the planes below in the 3d viewport after importing, the ones on the left have been spaced, while the ones on the right are in the default position.




This is how they look without Shadeless enabled when rendered.



And this is how they look with Shadeless enabled when rendered.